diff --git a/plugin/manager.go b/plugin/manager.go
index 438cd1eb9..f1b398f4f 100644
--- a/plugin/manager.go
+++ b/plugin/manager.go
@@ -192,7 +192,9 @@ func (m *Manager) RemoveUser(userID uint) error {
 				return err
 			}
 		}
+		m.mutex.Lock()
 		delete(m.instances, pluginConf.ID)
+		m.mutex.Unlock()
 	}
 	return nil
 }